Answer:
Yes, you do have to pay an agent, but not in the way you are thinking. They cost about 15% of your earnings, meaning if they are asking for money before you sell, don't hire!!!!
Consider this, too:
Their job is to get YOU jobs, so "the ball starts in their court," as it were. You owe an agent nothing until they get you work, so a good one will work very hard on your behalf. Then, as stated above, they get a percentage. The more jobs they get you (and the higher the jobs pay), the more they make.
Once upon a time, 10% was the standard cut for an agent, so unless you're desperate for their services, start there.
Your agreement could also include an "incentive clause." Negotiate with them for 10-12% until your annual income exceeds an agreed-upon amount (say, for example, 100,000 a year), at which time their cut would bump up to 15%. If you do work in incentives, include a "reverse incentive," meaning that if your income drops back BELOW the agreed-upon amount, their percentage drops back down, too.